A fastening tool is known which is specifically designed for fastening a bolt or a nut having a relatively large nominal diameter and which is capable of outputting a high torque and has a reaction receiver. Generally, however, such a dedicated fastening tool is heavy and expensive. Therefore, an attachment is available which has a power increasing device and which can be removably attached to a fastening tool having a lower output torque than the dedicated fastening tool. In Japanese Utility Model No. 3129070, for example, a fastening aid power unit is disclosed which is configured to be removably attached to an electric driver and has a planetary gear power increasing device and a reaction receiver.